1. Field of the Invention
This invention relates to an internal combustion engine of the split type including active cylinders being always active and inactive cylinders being inactive below a given engine load and, more particularly, to such an engine having in its exhaust passage an exhaust gas sensor for feedback control to ensure that the fuel supplied to the engine is correct to maintain a desirted optimum air/fuel ratio.
2. Description of the Prior Art
It is generally known that internal combustion engines demonstrate higher fuel combustion and thus higher fuel economy when running under higher load conditions. In view of this fact, split type internal combustion engines have already been proposed as automotive vehicle engines or the like subjective proposed as automotive vehicle engines or the like subjective to frequent engine load variations. Such split type internal combustion engines include active cylinders being always active and inactive cylinders being inactive when the engine load is below a given value. At low load conditions, the flow of fuel and air to the inactive cylinders is cut off so that the engine operates only on the active cylinders for relatively increasing active cylinder loads resulting in high fuel economy.
A split type internal combustion engine has been proposed which is associated with an exhaust gas recirculation system for re-introduction of a great amount of exhaust gases into the inactive cylinders to minimize inactive cylinder pumping losses during a split engine operation and also with an air/fuel ratio sensor adapted to provide a feedback signal for maintaining the air/fuel ratio of the mixture in each cylinder at the stoichiometric value. Such a split type internal combustion engine exhibits much higher fuel economy.
One difficulty with such conventional split type internal combustion engine is that the exhaust gas sensor is exposed to the exhaust gases re-introduced into the inactive cylinders and discharged therefrom while the engine is operating in a split cylinder mode of operation under low load conditions. This causes a reduction of the temperature of the exhaust gas sensor to spoil its performance and also provides previous air/fuel ratio indicative information to the exhaust gas sensor resulting in improper air/fuel ratio control.